1. Field of the Invention
The invention relates generally to a technique for sensing disturbances such as shear waves, pressure changes, acoustic signals, etc. and, more particularly, but not by way of limitation, it relates to method and apparatus using elliptically polarized light waves to derive an electric field vector that is indicative of the change in length of a single-mode polarization-preserving fiber-optic cable.
2. Description of the Prior Art
Pior art includes a number of disturbance sensing systems that utilize fiber-optic sensing elements. Such systems of recent development use fiber-optic sensors to provide an indication of the change in length of a single-mode fiber cable that is influenced by the environment to exhibit a change in the phase of a light travelling through it. Light leaving the end of the fiber cable is then sensed for combination with light coming from a reference fiber cable that was isolated from the environment. Combining of these two light waves then results in a high or low value of light due to constructive or destructive interference, and the resulting light level is related to the pressure that is exerted on the fiber-optic cable by the environment. U.S. Pat. No. 4,420,260 provides a representative teaching of the general class of phase-change sensing devices.
U.S. Pat. No. 4,534,222 teaches the use of a dual fiber coil interferometer with sensing and reference coils which develops a differential light delay, i.e., a phase difference, through the sensing optics in response to an external pressure. The detected phase difference is then directly related to the applied pressure or vibration and can be so indicated. U.S. Pat. No. 4,564,289 provides yet another teaching of differential measurement that utilizes two optical fiber coils experiencing difference in stress. The device utilizes two optical fibers arranged in series with a polarization rotator intermediate the coils so that equal stresses to the optical fiber will have equal and opposite effects upon polarized light transmission.
Finally, a U.S. Pat. No. 4,644,153 discloses an optical sensing equipment that utilizes first and second light sources that are alternately activated to experience a disturbance with subsequent determination of first and second detected outputs. The ratio of the two outputs is then calculated to provide a signal representing the measured quantity while the system eliminates the effects of light source fluctuations and fiber transmission losses by a process of common mode rejection.